1. Field of the Invention
This invention relates to a light emitting semiconductor device, and more particularly to a light emitting semiconductor device capable of simultaneously emitting light of different colors for generating combined light of a desired color. The present invention also relates to a method of making such a semiconductor device.
2. Description of the Related Art
In recent years, various light emitting semiconductor devices have been developed. In particular, a blue light emitting diode (blue LED) with a high brightness has been made available, wherein gallium nitride (GaN) compound crystal layers are successively grown on a sapphire substrate by organic metal chemical vapor deposition (hereafter abbreviated as "MOCVD").
Specifically, a GaN type blue LED comprises a sapphire substrate on which is a GaN buffer layer is formed, and a crystal layer laminate is formed on the buffer layer. The laminate includes two N-type semiconductor layers (GaN layer and AlGaN layer), a light emitting semiconductor layer (InGaN layer), and two P-type semiconductor layers (AlGaN layer and GaN layer). A cathode electrode is formed on the N-type GaN layer, whereas an anode electrode is formed on the P-type GaN layer.
Conventionally, blue LEDs have found limited use only as a blue light source. However, since the GaN type blue LED described above has been found to provide a sufficient brightness, it can be combined with a green LED and a red LED to generate combined white light. This has opened up the possibility of using a group of LEDs as a white light source for a reflective backlight type liquid crystal display or a transmission type liquid crystal display. Compared with a fluorescent lamp which has been conventionally used as a light source for a liquid crystal display, the use of grouped LEDs provides a great reduction in the overall size and weight of the display while drastically prolonging the service life.
However, the use of grouped LEDs (blue LEDs, green LEDs and red LEDs) as a white light source is still disadvantageous for the following reasons.
First, different kinds of LEDs need be separately mounted on a circuit board, which increases the time required for mounting.
Secondly, the color of the combined light actually recognized by the viewer depends on the number and arrangement of the different kinds of LEDs mounted on the circuit board. Thus, it is very difficult and time-consuming to make adjustment as to the number and arrangement of the different kinds of LEDs at the time of mounting them onto the circuit board. If the adjustment is improper, the resulting combined light may deviate slightly from the intended white color or any other intended color.
In the third place, the checking as to the color of the combined light actually generated by the grouped LEDs need be performed after the LEDs are mounted on the circuit board. Thus, if the combined light is subsequently found to deviate from the intended color, one or more of the LEDs must be replaced by new one or ones, thereby resulting in economical losses.